In utero transplantation is currently being investigated as therapy for patients with lethal congenital immunodeficiency disorders. Partial immunoreconstitution has been observed in a patient with a severe combined immunodeficiency disorder (scid) by in utero and post-natal transfer of fetal liver and fetal thymus and in a patient with bare lymphocyte syndrome (BLS), an inherited disorder in which major histocompatibility complex (MHC) class I and/or class II antigens are not expressed or expressed at low levels. IL-2 deficient humans have normal numbers of T- cells which are dysfunctional. These 3 types of immunodeficiency disorders span the spectrum of immunological abnormalities including deficiencies in mature T-cells number (scid), subtypes of mature T-cells (BLS), T-cell dysfunction with normal T-cell numbers (IL-2 deficiency), B- cells number (scid), and primary (BLS) or secondary (IL-2 deficiency) B- cell dysfunction. We propose to improve upon these results using the most representative murine models including strains of mice that have scid, BLS with deficiencies in MHC class I and/or class II, and IL-2 deficiency. While engraftment has been observed in fetal recipients of in utero transplanted hematopoietic cells, the level of engraftment in has generally been low. In this proposal, we will answer the following: 1. Is the donor pluripotent hematopoietic stem cells (PHSC) number limiting for in utero engraftment?; 2. Will committed progenitor cells facilitate engraftment or provide early post-natal reconstitution?; 3. Are adult BM progenitors at a competitive hematopoietic disadvantage to resident fetal PHSC, can we increase their proliferative capacity in utero, and is proliferation dependent upon the site of localization of donor cells? Donor PHSC enrichment techniques will be used to concentrate PHSC and/or committed progenitor cells from one type of donor graft that can be competed in vivo with the second type of donor graft and the fetal recipient. In utero migration of adult versus fetal PHSC will be examined using in situ immunohistochemistry and FACS, while early post-injection proliferation in situ will be assessed by using lipophillic dyes that irreversibly bind to lipid bilayers in the cell membrane by immunohistochemistry and by FACS analysis of engrafted cell populations. We will attempt to increase engraftment by preincubating the enriched donor BM fractions with cytokines in a stroma non-contact or a stroma-free setting to diminish the effect of negative regulators on progenitor cell proliferation. In specific aim 2, we will apply these strategies to the immunodeficiency setting and answer the following questions: 4. Can we achieve multilineage engraftment in non-anemic mice with lethal immunodeficiency syndromes? 5. Will thymic epithelial cell engraftment (required for BLS correction) be possible in utero? 6. What will be the extent and specificity of immune reconstitution in partial or fully engrafted immunodeficient mice? 7. How will fetal tolerance induction by in utero transfer differ from classical neonatal tolerance induction?